In the heat treat field, metal work is to be heated and cooled in accordance with known, time-temperature-atmosphere composition heat treat processes. Simplistically, the work is heated, held and cooled at specific rates and times while the gaseous or furnace atmosphere surrounding the work is controlled to impart desired metallurgical and mechanical properties to the work.
In the furnace art, cooling of the work (except for furnace cool heat treat processes) always occurs by convection, while heating by convection is typically limited to low temperature furnace applications, about a maximum of 1400.degree. F. Convective heat transfer is typically accomplished in batch furnaces by either baffle arrangements which divert and direct the flow of furnace atmosphere about the work or, alternatively, by high speed jets which are used to impinge the work to establish high heat transfer rates. All baffle arrangements require adjustment and are thus "sensitive" to performing different processes on different furnace loads. In addition, the cost to construct the baffles is expensive. Jet nozzle arrangements are generally used only for cooling the work and are specifically designed as a predetermined nozzle configuration for impinging cooled atmosphere against a specific workpiece shape or are of a general configuration which directs multiple streams of jets against the work. In either instance, separate longitudinally-extending plenum chambers are built within the furnace to develop high pressure jets or alternatively an external heat exchanger is used which then pumps the cooled air into a plenum or manifold distributor within the furnace. This is also an expensive furnace construction.
There are, however, numerous, convective heat transfer arrangements in the prior art and it is known to use the intake of a fan as a centrally positioned under-pressure zone to establish closed loop, pump type recirculation schemes in the sense that atmosphere is drawn into the fan, pressurized by the fan in a plenum chamber and then directed through a manifold to impinge the work. A variation of this theme is disclosed in U.S. Pat. No. 4,789,333 to Hemsath (incorporated herein by reference) where a free-standing, longitudinally moving circular jet is developed through an orifice and expanded into turbulent contact with a cylindrical shell member as the jet travels the length of a cylindrical shell. At the end of the shell, the high speed jet is redirected by a special diverter plate to impinge the work and after impingement the atmosphere is collected through the under-pressure zone to be pressurized again into a jet. This arrangement is limited to the thin shell furnace of the '333 patent which can be placed into heat transfer contact with a high velocity jet travelling along its length. U.S. Pat. No. 4,395,233 to Smith et al (incorporated herein by reference) illustrates the use of a central under-pressure zone to cause recirculation of flat sheets of forced air in a baking oven by causing the forced air to assume a torroidal shape as it travels to the fan under-pressure zone. However, Smith's oven is rectilinear in configuration and Smith uses the same prior art concept of pressurizing the wind in a plenum chamber which is directed from the plenum chamber through rectangular slots which orifice the forced air into his oven. None of the recirculation arrangements is sufficient to develop the "wind" pattern required in the heat treat furnace applications to which the present invention is concerned.
Also, it often occurs that the work, when heated, emits toxic gases or fumes. For example, powdered or sintered metal parts when heated, even at low temperatures, emit smoke which contains hydrocarbons and require separate afterburners or incinerators to burn the hydrocarbons and other pollutants which increases furnace cost. Heat is typically recovered from the incinerators through heat exchanges and typically used to heat boilers or preheat the combustion air used in the furnace burners. This is inefficient because heat must first be developed to incinerate the volatiles and the recovery of the heat is limited to secondary processes.
Apart from specific furnace design considerations, in general, furnace construction is typically divided between low temperature and high temperature applications. As indicated previously, heat transfer efficiencies dictate that low temperature furnaces be heated by convection while high temperature furnaces heat the work by radiation although convection/radiation heat transfer is employed to heat the work through the lower temperature ranges. For high temperature applications, the furnace construction is further divided between those furnaces which operate at slight positive pressure, or standard atmosphere furnaces, and those furnaces which operate under vacuum such as vacuum furnaces, ion "glow discharge" furnaces, etc. Traditionally, high and low temperature, positive pressure, batch furnaces were typically distinguished in their construction by the type of insulation used in the furnace. Low temperature applications in many instances use an "oven" panel construction where low grade insulation is simply sandwiched between metal skins to form panels which are welded together to form a box into which a burner is placed. In contrast, high temperature, standard atmosphere furnaces typically were constructed about a steel liner or casing to which refractory was bricked. With improvements in ceramic, fibrous furnace insulation which replaced refractory brick linings, the physical distinctions between the two furnace constructions began to dissipate although the low temperature furnace, because of insulation prices, remains a low cost furnace while high temperature applications use a more expensive insulation construction technique. Finally, certain metallurgical processes require that the heat treating gas be diffused into the case of a heated metal part under a vacuum. Typically, vacuum furnace constructions use a double wall or double casing construction which is spherically or cylindrically shaped to withstand collapse when a vacuum is pulled therein and water is typically circulated between the walls to provide a cold wall design so that the furnace door can have an elastomer seal to vacuum seal the enclosure. Insulation is provided on the inside of the inner casing. There are, however, heat treat applications which fall short of the high vacuum levels that would demand a double walled, vacuum vessel construction.
While a high temperature furnace can be used to perform either high or low temperature heat treat processes, the use of a high temperature furnace to perform low temperature heat treat processes is not cost effective to the heat treater. Further, it is not cost effective to the heat treater to use a vacuum furnace to perform a high temperature heat treat process such as carburizing when the carburized case tolerances are such that the process could be performed in a standard atmosphere, high temperature batch furnace. Basically, the furnace throughput coupled with furnace cost dictate the heat treater's charge and heretofore precluded small heat treaters who did not have a range of furnaces from competing with large heat treaters who could afford to purchase a number of different furnaces to perform different heat treat processes.